There are two basic groups of can lining compounds in use today; solvent based and water based.
Solvent based compounds comprise a rubber based can lining compound dissolved and/or dispersed in one or more solvents. After the compound is lined onto a can end, the solvent is driven off to leave a resilient gasket.
Water based compounds are similar, however the compounds are dispersed and/or emulsified in water rather than a solvent. These materials therefore avoid the problems normally associated with solvents, such as pollution, flammability, and health effects.
Typically, both compounds are supplied under pressure to a lining nozzle. A can end, mounted to a rotary chuck below the nozzle, is rotated at a set speed. The nozzle is opened for a length of time sufficient to apply the required amount of compound to the can end.
The key to obtaining an acceptably lined can end is the sufficient and consistent deposition of compound as defined by the film weight and placement of the compound on the end. Film weight is the amount of compound that is applied to each can end. If too much is applied, the excess compound could distort the seal causing leakage, compound is wasted and the profitability suffers. If too little is applied, the end is rejected as it will not form a proper seal.
Film weight can be approximately determined by the following equation: ##EQU1## where FW is film weight, P is the pressure difference between that contained within the lining system and the atmospheric air pressure, D is the diameter of the lining nozzle orifice, .eta. is the viscosity of the compound as it flows through the nozzle, L is the lead length of the nozzle, and t is the lining time. This equation has been simplified and does not take into account other variables such as the wear of the nozzle, the configuration of the inside surface of the nozzle (tapered, etc.), the height to which the nozzle needle is lifted during lining, transient flow response from needle opening/closing, and elastic response of the compound. However, these variables are secondary in their effect and can, for purposes of this discussion, be ignored.
For a manufacturer of can ends, D and L are fixed for a given nozzle. Pressure and viscosity tend to vary depending on environmental effects on the system. Pressure and viscosity must be controlled and held constant in order to obtain consistent and sufficient film weights.
In solvent systems, the pressure and temperature are regulated by a "conditioner". This conditioning system comprises a rotary gear pump, a filter and a heater connected to the supply side of a lining nozzle and a back pressure regulator or pinch valve connected between the exit side of the nozzle and the gear pump. Gear pumps have steady flow and discharge pressure. Constant pressure in the system is required to maintain consistent film weight. In order to preclude a fluctuation in pressure each time the lining nozzle is opened, the recycle system is designed to recirculate large amounts of compound, usually 40 to 50 times the amount discharged through the nozzle. The heater is thermostatically regulated such that the temperature of the compound in the conditioner is closely controlled. As a consequence the compound viscosity, which is temperature sensitive, is accurately maintained. If the lining system should become inoperative for an extended period of time, the compound could make hundreds of passes through the recycle system. This amount of recycling is not a problem for solvent based compounds which are generally thermodynamically stable products.
However, water based compounds, being emulsions and/or dispersions, are not thermodynamically stable. Water based compounds require the use of surface active agents or protective colloids to maintain the compound in a usable state until lined.
The close clearances associated with rotary pumps cause a great amount of shear stress on water based compounds which destabilizes the compound, thus precluding the use of a conditioning system as is used in solvent based compounds.
This inability to condition water based compounds causes problems to the can end manufacturer in controlling and obtaining a consistent film weight. Fluctuations in ambient temperature which frequently occur on a daily basis, shutdowns during which the viscosity of the compound increases and the inability to hold the compound at a constant pressure precludes can end manufacturers from controlling film weights at +/- 10% of the desired weight, which is the standard for solvent based compounds. Field experience has shown that controlling film weights even at +/- 15% of the desired weight is often difficult to achieve with water based compounds.
The present invention overcomes the difficulties encountered with lining water based compounds and greatly improves the film weight control of water based compounds.